I have found this:
Which channel is right for me?
You probably want to use the current stable, ie: nixos-18.03.
nixos-18.03 comes from the release-18.03 branch on GitHub. It maintains some package version stability over its six-month life, providing better guarantees that your system will keep working without major changes for the life of the release.
The stable branch also gets timely security patches delivered through channels.
The channels beginning with the nixos- prefix are thoroughly tested from the perspective of being an operating system. This means we test the X server, KDE, various servers, and lower level details like installing bootloaders and runnning the nixos installation steps. This is in contrast to the nixpkgs- channel prefix, where only limited testing is performed.
If you want to live on the bleading edge, perhaps nixos-unstable is right for you. It very frequently receives patches and updates that make larger version jumps than the stable branch.
The unstable branch also regularly receives security patches, however sometimes it can take several weeks for security patches to end up in the channels, due to test being broken.
If you are only using Nix as a package manager, perhaps the nixpkgs-unstable branch is better for you. This channel is very frequently updated, but does not receive the thorough testing the NixOS channels receive. DO NOT USE THIS FOR NIXOS.
from xLICENSE ยท GitHub